Microbiology

Question 1

In a quality control procedure on a new batch of Mueller-Hinton plates using a stock culture of Staphylococcus aureus (ATCC 25923), all the disk zone sizes are too small. The most likely reason this is that the:

Answer 1

bacterial suspension was not diluted to the proper concentration

Question 2

Candida albicans and Candida tropicalis can be used as positive and negative controls, respectively. True or False?

Answer 2

TRUE

Question 3

Describe the oxygen requirement, Gram stain, and formation of spores of Bacteroides.

Answer 3

anaerobic, Gram-negative nonsporulating

Question 4

Does the type of water vegetation consumed aid in the identification of a stool specimen that was observed for ova and parasite examination that contained numerous rhabditiform larvae?

Answer 4

no

Question 5

Gram stain examination of a CSF specimen indicates the presence of yeast-like cells with Gram-positive granular inclusions. Which technique should be used next to assist in the identification of this organism?

Answer 5

India ink

Question 6

How is Histoplasma capsulatum most frequently transmitted?

Answer 6

inhalation

Question 7

The appropriate specimen for the diagnosis of mucormycosis is?

Answer 7

eschar biopsy

Question 8

A gamma-hemolytic Streptococcus that blackens bile esculin agar but does not grow on 6.5% NaCl broth is most likely:

Answer 8

group D Streptococcus

Question 9

The procedure that assures the most accurate detection of mecA-mediated oxacillin resistance in routine broth microdilution susceptibility testing against S. aureus is:

Answer 9

use of cefoxitin for testing

Question 10

What is the most common sign of Enterobius vermicularis infection?

Answer 10

intense perianal itching

Question 11

Which feature distinguishes Erysipelothrix rhusiopathiae from other clinically significant non-spore0forming, gram-positive, facultatively anaerobic bacilli?

Answer 11

H₂S production

Question 12

An urethral swab obtained from a man with a urethral exudate was plated directly on chocolate agar and modified Thayer-Martin agar, and a Gram stain was made. The Gram stain showed gram-negative diplococci. The culture plates were incubated at 35°C, but had no growth at 48 hours. The most likely failure for organism growth is that the:

Answer 12

organism requires CO₂ for growth

Question 13

An important cause of acute exudative pharyngitis is:

Answer 13

Streptococcus pyogenes

Question 14

Clinical resistance to penicillin correlates most frequently with beta-lactamase production in:

Answer 14

Neisseria gonorrhoeae

Question 15

Colds and other acute respiratory disease are most often associated with:

Answer 15

adenovirus

Question 16

Stool culture gives the following reactions: colorless on MacConkey; yellow-orange on HE; A/A, no gas, no H₂S on TSI; urease positive. These screening reactions are consistent with which enteric pathogens?

Answer 16

Yersinia enterocolitica

Question 17

An organism that exhibits the satellite phenomenon around colonies of Staphylococci is:

Answer 17

Haemophilus influenzae

Question 18

A penicillin-resistant Neisseria gonorrhoeae produces:

Answer 18

beta-lactamase

Question 19

Lab workers should always work under a biological safety hood when working with cultures of:

Answer 19

Coccidioides immitis

Question 20

A gram-positive coccus isolated from a blood culture has the following characteristics: optichin negative; bacitracin negative; bile esculin negative; hippurate hydrolysis positive; catalase negative. This organism is most likely:

Answer 20

Streptococcus agalactiae

Question 21

Susceptibility testing is performed on a Staphylococcus aureus isolate from a blood culture with the following results: oxacillin resistant; cefazolin susceptible; clindamycin susceptible; erythromycin susceptible; trimethoprim/sulfamethoxazole susceptible; vancomycin susceptible. What should the technologist do next?

Answer 21

the cefazolin result should be changed to resistant since the oxacillin result is resistant

Question 22

A urine Gram stain shows gram-positive cocci in clusters. The organism tested catalase positive. To speciate this organism from culture, the technician should perform a coagulase test and a/an:

Answer 22

novobiocin susceptibility

Question 23

The following results were obtained from a culture of unknown origin: gram-negative diplococci; indophenol oxidase positive; glucose positive; maltose negative; sucrose negative. The most likely source of the specimen would be the:

Answer 23

genitourinary tract

Question 24

The organism most commonly associated with neonatal purulent meningitis is:

Answer 24

group B Streptococci

Question 25

A beta-hemolytic *Streptococci* that is bacitracin-sensitive and CAMP-negative is:

Answer 25

group A *Streptococcus*

Question 26

A 73 year-old man diagnosed as having pneumococcal meningitis is not responding to his penicillin therapy. Which of the following tests should be performed on the isolate to best determine this organism's susceptibility to penicillin?

Answer 26

oxacillin disk diffusion

Question 27

MacConkey media for screening suspected cases of hemorrhagic *E. coli* O157:H7 must contain:

Answer 27

sorbitol

Question 28

An antibiotic used to suppress or kill contaminating fungi in media is:

Answer 28

cycloheximide

Question 29

The most frequent cause of prothetic heart valve infections occurring within 2-3 months after surgery is:

Answer 29

*Staphylococcus epidermidis*

Question 30

A branching gram-positive, partially acid-fast organism is isolated from a bronchial washing on a 63-year-old woman receiving chemotherapy. The organism does not hydrolyze casein, tyrosine, or xanthine. The most likely identification is:

Answer 30

*Nocardia asteroides*

Question 31

The disease-producing capacity of *Mycobacterium tuberculosis* depends primarily upon:

Answer 31

capacity to withstand intracellular digestion by macrophages